1. Field of the Invention
The present invention relates to a magnetic recording medium in a tape form to be used in video and audio appliances or information related equipment. More specifically, the invention relates to a method for manufacturing a coated type magnetic recording medium possessing an excellent surface smoothness and a high squareness ratio in the longitudinal direction of the tape, in order to realize a high orientation by the minimum limit of equipment and power by the most effective orientation technique.
2. Description of the Prior Art
Various recording media are presently subject to a demand that the recording and reproducing characteristics in high density recording region be enhanced along with the trend of high picture quality, digital processing of signals, down-sizing of appliances, and high speed processing.
In a coated magnetic recording medium, the key for improving the recording and reproducing characteristics in the high frequency region is to enhance the magnetic characteristics of the medium layer and smooth the medium surface. Enhancing the magnetic characteristics is intended to increase the magnetic intensity of recorded signals. Smoothing the medium surface is intended to lessen the spacing loss occurring at the time of recording and reproducing by suppressing the gap between the record/play head and magnetic particles as far as possible at the medium side.
In such background, in the field of recording materials, those with higher magnetic energy have been studied, and iron oxide particles with improved coercive force by coating with Co and alloy particles high in saturated magnetization and coercive force are becoming the main type.
In the aspect of the process for applying and forming the shape of a magnetic particles and magnetic layer, research has been conducted into the technique for raising the packing density of the magnetic layer by reducing the sizes of the magnetic particles, lowering the axial ratio, and optimizing the composition ratio of the magnetic layer, a technique for aligning the array directions of magnetic particles in the machine direction to the limit by fortifying the orientation process, and a technique for optimizing the calendering conditions at the time of forming the magnetic layer and glass transition temperature of the binder resin.
Along with the improvement of magnetic particles, however, as the coercive force is increased, it is necessary to intensify the external magnetic field for fixing the magnetic moment in the initial phase of orientation in the easy magnetization axis direction, or when the saturated magnetization is increased, the magnetic repulsive force among magnetic particles tends to increase when arrayed. Besides, reduction of particle size brings about decrease of distance between particles after coating or in the paint state, and lowering of the axial ratio leads to a decrease of magnetic anisotropic energy of particles.
Thus, magnetic and morphological improvements of magnetic particles act negatively for obtaining an excellent squareness ratio. In the prior art, it has been attempted to cope with the higher coercive force, lower axial ratio and stronger repulsive force among particles by early drying of the coated film during the orientation process as well as by a powerful orientation magnetic field. As examples, various efforts have been made such as, further advancing the known art of orientation processing while drying (Japanese Laid-open Patent No. 59-71133), orientation after drying to such an extent that reaggregation does not occur magnetically, and then re-drying (Japanese Laid-open Patent No. 56-36496), or magnetic orientation means installed in the drying process (Japanese Laid-open Patent No. 60-70532), or preliminary orientation and drying followed by intermediate drying with simultaneous regular orientation, and then regular drying (Japanese Laid-open Patent No. 60-76023).
To obtain superior surface properties, a smooth and homogeneous coated film is required before calendering, and indispensable conditions are an orientation process and drying process which will not to cause roughness of the coated surface, as well as sufficient leveling after applying magnetic paints. In the conventional method, therefore, although the orientation may be improved, the coated film surface after drying is not excellent, and it is difficult to enhance the surface smoothness of the final magnetic layer.